WASHINGTON - The chairman of a House Appropriations subcommittee probed NASA Administrator Charles Bolden on Tuesday morning citing apparent "massive overuse" of premium first- and business-class air travel upgrades for agency employees, including trips first publicized by Scripps News nearly a month ago.

"I don't travel first class," Bolden told the committee.

Among the trips cited by Rep. Frank Wolf, R-Va., was a $14,773 flight NASA reported booking for Ames Research Center Director Simon "Pete" Worden to travel first class from Washington, D.C., to San Francisco in 2011.

When Scripps News first raised questions about the trip, NASA officials said its records were incorrect and the trip never happened. Worden told Scripps News it did.

Wolf also asked why Bolden charged taxpayers $1,641 for a one-way first-class flight from Washington, D.C., to Los Angeles in June 2010. NASA records say the average coach fare for the flight is less than $200.

Bolden said he often uses frequent flier miles to upgrade at no cost. He was unable to explain why the costlier fares were reported in NASA records, and said he would get answers.

"I can't help how it's accounted for," said Bolden, who was testifying about NASA's 2015 budget request before the House Appropriations Subcommittee on Commerce, Justice, Science and Related Agencies. "We are looking into it. The IG (inspector general) is looking into it."

Wolf, who chairs the subcommittee, cited "serious problems" with some of the space agency's fiscal decisions, including travel spending reported in annual disclosures to the General Services Administration. He urged more stringent belt-tightening and oversight.

"If NASA is going to continue to receive the support of Congress, it absolutely must do a better job," Wolf said. The criticism came as Bolden answered questions about the agency's $17.46 billion budget request.

NASA approved one-way premium upgrades that it reported cost from $3,000 to more than $10,000 apiece on 515 occasions over four years, Scripps reported after reviewing agency records obtained through the Freedom of Information Act.

Federal regulations say travelers flying at taxpayer expense must use the same spending restraints as "a prudent person would exercise if traveling on business." They require federal employees to fly coach for both domestic and international trips unless agencies can justify upgraded travel from among a few exceptions, for example a trip lasting longer than 14 hours.

Even when eligible for exceptions, some agencies ban employees from flying first or business class, which often costs taxpayers thousands of dollars more than a comparable coach ticket. The U.S. Trade and Development Agency says that except for some trips by its director, it has banned upgrades altogether and achieved substantial savings.

"As a foreign assistance agency we spend a lot of time going to Africa and Asia (all over 14 hours) and fly economy," agency spokesperson Tom Hardy wrote in an email to Scripps. "This has allowed USTDA staff to increase the number of visits to the region to provide greater oversight of projects,"

In some cases, NASA reported to the GSA that Bolden not only upgraded his own international ticket, but those of his wife, Alexis, as well. NASA said Mrs. Bolden had been officially invited by the host countries.

For 12 flights taken by Bolden, NASA cited the 14-hour rule to justify why he traveled premium instead of coach. The agency wrote to Scripps on March 4 of this year that it was still researching what exceptions would have justified three other upgraded flights Bolden took, including his Washington-to-Los Angeles flight.

"I would be very happy to show anyone my travel records," he told members of the subcommittee.

While Bolden testified that the agency had "tried for months" to work on getting Scripps the information, NASA has denied a freedom of information request from the news organization requesting the justification for Bolden's Washington-to-Los Angeles flight and two others. NASA has declined to respond to subsequent emails. Bolden declined previous requests for interviews from Scripps and would not answer questions following the hearing.

NASA also has acknowledged errors made "throughout" years of required annual reports of its travel disclosures to the General Services Administration. In 2012, NASA didn't disclose any of its premium upgrades.